Direct gap III-V materials such as gallium arsenide are attractive candidates for making high efficiency solar cells due to their strong absorption properties. The fabrication of high efficiency III-V solar cells can be achieved by epitaxial growth of the structures using various techniques such as metal organic chemical vapor deposition and molecular beam epitaxy.
Referring to FIG. 1A, a conventional single junction III-V solar cell structure 10 is shown. This structure includes a base 12 positioned between an emitter layer 14 and a back surface field (BSF) layer 16. The base in the structure 10 is Zn:InGaAs, the emitter is Si:InGaAs and the BSF layer is Zn:InGaP. A window layer 18 and contact 20 are positioned above the emitter, the window layer being a ternary III-V material such as Si:In0.5Al0.5P and the electrically conductive contact being Si:In0.01Ga0.99As. Ternary III-V materials help to minimize light absorption in the window layer. A p-type Ge substrate 22 oriented 6° towards <111> is positioned at the bottom of the structure 10. This conventional structure 10 requires the growth of a buffer region between the substrate and BSF layer 16. In this exemplary structure 10, the buffer is comprised of a Zn:In0.01Ga0.99As layer 24A adjoining the BSF layer 16 and a Zn:In0.5Ga0.5P layer 24B between and adjoining the top surface of the substrate 22 and the bottom surface of the top layer 24A of the buffer region 24. The buffer is provided in order to minimize antiphase boundaries (APB) defects that are formed while growing compound III-V materials on elemental substrates, such as germanium.
A conventional solar cell structure 30 including middle, top and bottom cells is shown in FIG. 1B and can be referred to as a tandem structure. In this exemplary structure 30, the middle cell is comprised of a p-Ga(In)As absorber layer or base 32, a n-Ga(In)As emitter layer 34 adjoining the top surface of the base 32, a window layer 36 adjoining the top surface of the emitter layer 34, and a BSF layer 38 adjoining the bottom surface of the base 32. The top cell is comprised of a p-GaInP base 40, an n-GaInP emitter layer 42, a window layer 44 and a BSF layer 46. A wide band gap tunnel junction 48 is provided between the top and middle cells. A germanium bottom cell includes a p-Ge base 50 and a n+Ge emitter layer 52. A n-Ga(In)As buffer 54 is positioned between the emitter layer 52 of the bottom cell and a second wide band gap tunnel junction 48 that adjoins the middle cell. A nucleation layer 53 is formed between the emitter layer 52 and buffer 54. Top and bottom electrical contacts 56, 58 allow the structure 30 to be electrically coupled to other devices. A n+Ga(In)As layer 60 is provided between the top contact 64 and the window layer 44 of the top cell. An antireflective coating 62 adjoins the window layer 44 of the top cell.
Solar cell structures can be initially grown in reverse order as shown in FIG. 2 to enhance functionality without performance degradation. In other words, the emitter layer may be provided near the bottom of the cell while the BSF layer is at or near the top. The solar cell structure 70 shown in FIG. 2 includes many of the same layers employed in the structure 10 shown in FIG. 1. The same reference numerals are employed to designate such layers. The buffer region 74 in the inverted structure 70 is doped with silicon as opposed to zinc. As indicated in the figure, the buffer layer 74A is formed of Si:In0.01Ga0.99As and is 2.0 μm in thickness while the optional nucleation layer 74B is formed of Si:In0.5Ga0.5P. An etch stop layer 76 is formed between the ohmic contact layer 20 and the buffer layer 74A. A contact layer (not shown) may be provided on the BSF layer 16. The fabrication of a III-V solar cell structure such as shown in FIG. 2 involves growing the layers on a substrate, removing the germanium substrate 22 and the layers between the substrate and ohmic contact layer 20, none of which are part of the active device, and then further processing to produce a finished device. A metal lead (not shown) can then be formed on the contact layer 20.
FIG. 2B schematically illustrates an exemplary prior art photovoltaic structure 80 that includes a top portion 82 of a solar cell structure. As discussed above with respect to FIG. 1A, the top portion of a single junction solar cell structure includes an absorber layer, an emitter, a window layer, and a contact layer. In the case of a multi junction cell, the top portion would include an absorber layer, an emitter, a window layer, a tunnel junction, and the top sub-cells. The structure 80 further includes a wide bandgap III-V BSF layer 84, a highly doped (In)GaAs layer 86, and a metal reflector and contact 88. Wide bandgap materials such as InGaP, InAlP, InGaAlP and AlGaAs are widely used as a minority carrier mirror to form back surface fields in III-V solar cells. Because of the challenges associated with forming low resistivity ohmic contacts on wide bandgap materials, a highly doped layer of (In)GaAs with low indium content follows the deposition of the wide bandgap material to facilitate formation of the ohmic contact.